The present invention relates to a logging-while-drilling tool and more particularly relates to a drive train in a logging-while-drilling tool which utilizes a motor-actuated rotary valve positioned in the drilling fluid to generate a pressure wave signal representative of a "logged" condition.
The desirability of a system which is able to measure downhole drilling parameters and/or formation characteristics and transmit them to the surface while actual drilling of an earth well is being carried out has long been recognized. Several such systems have been proposed and are commonly referred to as "logging-while-drilling" systems. In logging-while-drilling systems, one of the major problems exists in finding the communication link necessary for telemetering the information from a downhole location and having it arrive at the surface in a meaningful condition.
In this regard, it has been proposed to telemeter the desired information by means of an acoustical pressure wave signal generated in and transmitted through the circulating mud system normally associated with well drilling operations. The pressure wave signal which is representative of a downhole condition is generated in the mud downhole near the bit by a signal-generating means and the wave travels up the hole through the mud to a signal processor at the surface. One logging-while-drilling system utilizing this basic type of telemetry is disclosed and fully described in U.S. Pat. No. 3,309,656 to John K. Godbey, issued Mar. 14, 1967.
In telemetering downhole information by means of an acoustical pressure wave signal carried through the mud, as mentioned above, one important consideration is how the information is actually encoded into the continuous wave signal. One system for encoding information to such a signal is disclosed and fully described in U.S. Pat. No. 3,789,355 to B. J. Patton, issued Jan. 29, 1974. This system includes a rotary valve driven at a constant speed to produce a continuous phase-locked pressure signal in the mud stream. An electrical motor which drives the valve through a drive train responds to a signal representing a piece of measured downhole information to effect a phase shift in the pressure signal by speeding up or slowing down rotation of the valve.
The rotor and stator which form the rotary valve used in such logging-while-drilling systems inherently act as a mud turbine in the mud stream and, consequently, are subject to producing hydraulic torques which are a function of both mud parameters and valve design. It follows that the motor driving the valve must be capable of producing a maximum torque equal to the sum of (1) the torques generated due to the frictional losses in the downhole logging-while-drilling apparatus; (2) the hydraulic torque produced by the rotary valve; and (3) the torques required to maintain the speed of the valve and to accelerate and decelerate the valve to execute the necessary phase shifts. Where the rotary valve is designed to produce little or no hydraulic torque at nominal mud conditions, as is normally the case in the system described in U.S. Pat. No. 3,789,355, the drive train between the motor and the valve will normally include a speed reduction transmission which is highly efficient in transmitting torque in both directions, i.e., from the motor to the valve and from the valve to the motor. Such a drive train is disclosed in U.S. Pat. No. 3,705,603 to D. E. Hawk, issued Dec. 12, 1972.
Theoretically, however, it is more desirable to design the turbine features of the valve so that the valve itself produces sufficient hydraulic torque under nominal mud conditions to drive the valve at the speed necessary to produce the desired constant pressure signal. This, in effect, unloads the motor and leaves substantially all of the power of the motor available for acceleration and deceleration of the valve to effect the desired phase shifts of the constant pressure signal.
Unfortunately, however, when the valve is designed to produce the hydraulic torque necessary to give better motor operation, the valve also produces larger variations in said hydraulic torque as mud conditions change. With a speed reduction transmission of the type as shown in U.S. Pat. No. 3,705,603, these torque variations may generate substantial dynamic loads at the motor which, in turn, must be compensated for by the motor. Therefore, when the rotary valve is designed to generate positive torque to aid in driving the valve, it is desirable to prevent this positive torque from being transferred back through the drive train to the motor.